clc;
syms t f(t);
F=t^2*f*sin(t);
G=simplify(diff(F,t,3))

y1=simplify(subs(G,f,exp(-t)));

y2=simplify(diff(t^2*exp(-t)*sin(t),3))
result=y2-y1

% 上述两个代码做的是同一件事情，